This subject matter disclosed herein relates generally to physiological monitoring systems, and more particularly to systems and methods to extract physiological parameters from electrical impedance measurements.
Electrical Impedance Spectroscopy (EIS) measurements are used to classify and quantify the complex electrical properties of materials, such as those that comprise a region of a human body. These electrical properties are determined by applying an electrical current or voltage, and measuring a response voltage or response current on one or more electrodes at a surface of the material under test. The applied excitation and measured response are processed to generate an estimate of complex electrical impedance. This process may be done using a single excitation, or this process may be repeated using two or more excitations to produce a measurement of a complex electrical impedance distribution that varies with the applied excitation. Electrical impedance measurements obtained by EIS systems can be used for monitoring human physiological parameters. The measurements may be obtained by applying very small electrical currents or voltages, using for example skin-contacting electrodes, and measuring the resulting voltages on the same or on different skin-contacting electrodes.
The obtained electrical impedance signals are a measure of several parameters, including the geometry (e.g., length, area and/or volume) between and among the electrodes and the complex electrical conductivity in the tissues between and beneath the electrodes (e.g. organs, muscle, fat and/or skin). Because the measured impedance is sensitive to variations in geometry, patient motion and other extraneous signal and noise sources can result in undesirable effects that corrupt (e.g., interfere with) underlying signals of interest, resulting in inaccurate measurements. The signals of interest may include, for example, respiration rate, cardiac pulsatility, and other anatomical and physiological phenomenon.
Known systems for monitoring respiration activity by impedance measurement use a single impedance measurement between two electrodes. The underlying source of interest is the airflow into and out of the lungs and the interfering sources include heart motion, patient movement, and other unrelated physiological motion. These interfering sources are inseparable with temporal or spectral techniques using a single impedance measurement between two electrodes.
Algorithms are also known to switch between different pairs of electrodes to avoid interference sources and improve signal visibility. For example, one method includes switching between a chest electrode that captures breathing motion due to dominant chest muscles and an abdomen electrode that captures motion due to dominant diaphragm muscles in abdomen-breathers. This method suffers from inseparability of desired and interfering sources.
Other known algorithms use information from a plurality of electrodes and employ electrical impedance tomography techniques to generate a reconstruction of the conductivity distribution of the interrogated area or volume. These systems can generate an image of the lungs filling or emptying of air, but require a multiplicity of electrodes, typically 16 or more, and a significant computing system for forward modeling and/or data and image reconstruction.
Non-electrical methods are also known for continuously measuring the ventilation rate of a patient. These methods may be performed by measuring airflow through the airway by intubation, using a mask or by a sensor in the nose or mouth of the patient. These continuous measurement methods are uncomfortable for the patient and accordingly not widely used. Other, less-intrusive methods may be performed using methods such as motion sensors, accelerometers, pressure sensors, microphones, acoustic sensors, and/or plethysmographic bands. These less-intrusive methods are prone to interference and motion artifact that reduces their sensitivity to measuring the physiological parameters of interest and inhibits their widespread use.